The present invention provides a coating system that represents an improvement over the coating systems described in U.S. Pat. Nos. 5,409,163, 5,540,384, 5,582,348 and 5,622,752, the disclosures of which are hereby incorporated herein by reference. The coating system of the present invention can be used in the methods taught in these patents, and can also be used as described herein.
There is an increasing need in industry to apply coatings to selected areas on a substrate, for example applying protective coatings to printed circuit boards. In this example, a protective coating needs to be applied to soldered connections and not to other areas such as electrical connectors, heat sinks, light emitting diodes (LED) or displays. In many cases, the areas to be coated are immediately adjacent to the areas where coating cannot be applied. If coating is applied to the “no-coat” areas, the circuit board will not function properly due to the presence of the coating.
In many cases it is necessary to apply a masking material, such as tape, to the no-coat areas prior to application of the coating to ensure that these areas remain free of coating. Typically the masking is applied prior to application of coating and removed after the coating is dry. The application and removal of the masking material is time consuming and labor-intensive. The elimination of the masking process is desirable in a manufacturing process to reduce production time and reduce labor costs, especially for high volume production.
In recent years, automated coating systems have been developed for the application of coatings with the goal of eliminating the masking and de-masking process. The systems typically consist of a coating applicator for large areas, a coating applicator for small areas, a motion and positioning mechanism for the coating applicators and a system controller.
Typically spray valves or film coating heads are used for coating the large areas on the substrate quickly to minimize time required for coating application. However, typical large area coating applicators deposit the coating patterns with irregular edges. The coating cannot be applied close to the no-coat areas with these applicators due to the irregular edge of coating deposition. In order to reach the goal of elimination of masking another applicator is required to coat the smaller areas immediately adjacent to the no-coat areas of the substrate.
Typically dispensing valves that produce a bead or line of coating are used for coating small areas on the substrate. The dispensing valves are comprised of a piston actuated internal needle in seat arrangement. An air-actuated piston that raises the needle out of the seat is used to activate the coating flow. A stroke of the piston is manually adjusted to set the flow rate of the coating. The coating flows from the valve in the form of a bead or is directed through an external needle to the substrate.
The motion and positioning system is used to move both coating applicators over the substrate to apply coating to the desired areas. The system controller operates the motion and positioning system as well as the coating applicators and enables programs to be created and stored for various coating patterns.
These automated systems have been effective in certain circumstances but they do not produce a uniform coating application and have not completely eliminated the requirement for masking.
In many cases, it is desirable to apply a thin, uniform coating to a substrate, such as the application of protective coatings to printed circuit boards. For example, the coating thickness must be precisely controlled with the new water-based conformal coatings. If these coatings are applied at a greater thickness than is optimal for curing, cracks will form during the drying process, reducing the coating's protective properties. Spray coating applicators are capable of applying a thin coating on a substrate. To facilitate spray application and formation of a thin film on the substrate, the coating is typically formulated with a lower viscosity of less than 100 Centipoise. However, difficulties arise if the same low viscosity coating formulation is used in the dispensing valve. The dispensing valve produces a bead or line of coating on the substrate and the coating will tend to flow or wet out on the substrate unpredictably with low viscosity coating mixtures. To overcome this problem, higher viscosity coating formulations are used with the dispensing valves, which produce a thicker coating application than the spray valve. The use of a low viscosity formulation for the spray applicator and a higher viscosity coating formulation with the dispensing applicator results in a non-uniform coating on the substrate.
Additional limitations of dispensing valves include imprecise flow rate control, difficulty producing short coating lines segments, heavy coating deposition at the start and end of a coating segment and susceptibility to uneven or warped substrate surfaces. The coating flow rate on a dispensing valve is set with a manual screw adjustment to adjust the stroke of the piston. This manual adjustment is subjective and will necessarily produce different results each time to valve is adjusted.
The needle and seat arrangement of dispensing valves produces discontinuities as the flow starts and stops. This coupled with the head motion profile tends to produce heavy spots at the start and end of a coating segment and makes is difficult to create a short coating segment. This effect can somewhat be overcome with programming techniques for the system controller, but this process is tedious and not repeatable. To apply coatings accurately to very small areas or in straight lines with a dispensing valve, an external needle is required. To achieve optimal results, the outlet of the needle must be very close to the substrate, typically within 1 mm. If the substrate is uneven or slightly warped coating skips may result due to changing distance between the needle and substrate. Additionally, the needle has a tendency to contact previously applied coating and pull it along with it causing skips and smears. The needle is also subject to being damaged if it comes into contact with the substrate.
In summary, the limitations of current selective coating systems include:                (1) Inability to provide a uniform coating thickness for both large and small areas to be coated.        (2) The requirement to use different coating formulations for the coating applicator for the large areas and the coating applicator for the small areas.        (3) Manual adjustment of the coating flow rate, which produces inconsistent results, due to subjective process setup.        (4) Difficulty controlling the dispensing process due to unpredictable coating flow-out on the substrate, heavy spots ant the beginning and end of coating segments and susceptibility to irregular substrate surfaces.        